Voltage-nonlinear resistors

ABSTRACT

The invention relates to voltage-nonlinear resistors having nonohmic resistance due to the bulk thereof and more particularly to varistors, which utility as surge absorbers and D.C. stabilizers, comprising zinc oxide, bisumuth oxide, cobalt oxide, boron trioxide and at least one member selected from the group consisting of magnesium oxide, calcium oxide, barium oxide and strontium oxide.

United States Patent 1 Matsuura et al.

[4 1 Jan. 28, 1975 VOLTAGE-NONLINEAR RESISTORS [75] Inventors: Mikio Matsuura; Atsushi lga; Yasuo Wakahata, all of Osaka, Japan [73] Assignee: Matsushita Electric Industrial Co.,

Ltd., Osaka, Japan [22] Filed: Aug. 14, 1973 [2]] Appl. N0.: 388,169

[30] Foreign Application Priority Data Aug. l4, 1972 Japan 47-Xl643 [52] U.S. Cl 338/20, 252/518, 338/21 [51] int. Cl HOlc 7/10 [58] Field of Search 338/20, 21;

[56] References Cited FORElGN PATENTS OR APPLICATIONS 831.69] 1/1970 Canada 338/20 lrinmry I:'.\'amim'rC. L. Albritton Altar/10y, Again, or FirmWcnderoth, Lind & Ponack [57] ABSTRACT The invention relates to voltage-nonlinear resistors having nonohmic resistance due to the bulk thereof and more particularly to varistors, which utility as surge absorbers and DC. stabilizers, comprising zinc oxide, bisumuth oxide, cobalt oxide, boron trioxide and at least one member selected from the group consisting of magnesium oxide, calcium oxide, barium oxide and strontium oxide.

5 Claims, 1 Drawing Figure VOLTAGE-NONLINEAR RESISTORS Various voltage-nonlinear resistors such as silicon carbide varistors, selenium rectifiers and germanium or silicon p-n junction diodes have been widely used for stabilization of voltage of electrical circuits or suppression of abnormally high surge induced in electrical circuits. The electrical characteristics of such nonlinear resistors are expressed by the relation:

where V is the voltage across the resistor, l is the current flowing through the resistor, C is a' constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:

where V, and V are the voltages at given currents l, and respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the extent to which the resistors depart from ohmic characteristics.

Nonlinear resistors comprising sintered bodies of zinc oxide with or without additives and non-ohmic electrode applied thereto, have already been disclosed as seen in U.S. Pat. Nos. 3,496,5 l 2, 3,570,002, 3,503.02) and 3,689,863. The nonlinearity of such varistors is attributed to the interface between the sintered body of zinc oxide with or without additives and silver paint electrode, and is controlled mainly by changing the compositions of said sintered body and silver paint electrode. Therefore, it isnot easy to control the C-value over a wide range after the sintered body is prepared. Similarly, in varistors comprising germanium or silicon p-n junction diodes, it is difficult to control the C-value over a wide range because the nonlinearity of these varistors is not attributed to the bulk but rather to the p-n junction. In addition, it is almost impossible for those zinc oxide varistors mentioned above and germanium or silicon diode varistors to obtain the combination of C-value higher than 100 volt, n-value higher than l and high surge resistance tolerable for surge more than 100Ap.

On the other hand, the silicon carbide varistors have nonlinearity due to the contacts among the individual grains of silicon carbide bonded together by a ceramic binding material, i.e. to the bulk, and C-value is controlled by changing a, dimension in the direction in which the current flows through the varistors. In addition, the silicon carbide varistors have high surge resistance thus rendering them suitable as surge absorbers. The silicon carbide varistors, however, have a relatively low n-value ranging from 3 to 7 which results in poor surge suppression as well as poor D.C. stabilization. Another defect of the silicon carbide varistor as a DC. stabilizer is their change in C-value and n-value during D.C. load application.

There have been known, on the other hand, voltagenonlinear resistors of bulk type comprising a sintered body of zinc oxide with additives, as seen in U.S. Pat. Nos. 3,663,458, 3,632,529, 3,634,337, 3,598,763,

3,682,841, 3,642,664, 3,658,725 and 3,687.87l, and U.S. Patent copending application No. 29.4l6. These zinc oxide varistors contain, as additives, one or more combinations of oxides or fluorides of bismuth. cobalt. manganese, barium,'boron. magnesium, calcium, strontium, titanium, antimony chromium and nickel. and are controllable in C-value by changing the distance between electrodes and have an excellent nonlinear property in an n-value. The power dissipation for surge energy, however, shows a relatively low value compared with that ofthe conventional silicon carbide varistor. so that the change rate of C-value exceeds 20 percent after two standard surges of 8 X 20 usec wave form in a peak current of SOOA/cm are applied to said zinc oxide varistors of bulk type. Another defect of these zinc oxide varistors of bulk type is in their poor stability for DC. load, particularly in their remarkable decreases of C-value measured in a low current region such as 0.lmA and 0.0lmA after applying high DC. power. This deterioration in the C-value is unfavorable for a voltage stabilizer which devices require high accuracy and low loss.

An object of the present invention is to provide a voltage-nonlinear resistor having high n-value, high power dissipation for surge energy and high stability for DC load even in a range of current less than 0.lmA/cm? This object of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the FIGURE is a cross-sectional view through a voltagenonlinear resistor in accordance with the invention.

Before proceeding with a detailed description of the voltage-nonlinear resistor contemplated by the invention, its construction will be described with reference to the aforesaid figure of drawing wherein reference character 10 designates, as a whole. a voltagenonlinear resistor comprising, as its active element, a sintered body having a pair of electrodes 2 and 3 in an ohmic contact applied to opposite surfaces thereof. Said sintered body 1- is prepared in a manner hereinafter set forth and is any form such as circular, square or rectangular plate form. Wire leads 5 and 6 are attached conductively to the electrodes 2 and 3, respectively, by a connection means 4 such as solder or the like.

A voltage-nonlinear resistor according to the inven tion comprises a sintered body of a composition comprising, as an additive, 0.0l 10.0 mole percent of bismuth oxide (Bi O 0.01 to 10 mole percent of cobalt oxide (C0 0 0.01 to 5.0 mole percent of boron trioxide (B 0 and 0.01 to 5.0 mole percent of at least one member selected from the group consisting of magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), strontium oxide (SrO) and the remainder of zinc oxide (ZnO) as a main constituent, and electrodes applied to opposite surfaces of said sintered body. Such a voltage-nonlinear resistor has non-ohmic resistance due to the bulk itself. Therefore, its C-value can be changed without impairing the n-value by changing the distance between said opposite surfaces. According to the invention, said resistor has high n-value and high stability for DC. load.

The higher stability with respect to DC. load and surge pulses can be obtained when said additive consists essentially of 0.] to 3.0 mole percent of bismuth oxide (Bi O 0 to 3.0 mole percent of cobalt oxide (C0 0 0.0l to 5.0 mole percent of boron trioxide (B and 0.01 to 5.0 mole percent of at least one member selected from the group consisting of magnesium oxide (MgO), calcium oxide (CaO), barium oxide (B210) and strontium 0xide(SrO).

It has been discovered according to the present invention that the higher n-value and higher stability with respect to DC. load and surge power can be obtained when said additive consists essentially of 0.1 to 3.0 mole percent of bismuth oxide (Bi-,0 0.l to 3.0 mole percent of cobalt oxide (C0 0 0.01 to 5.0 mole percent of boron trioxide (B 0 0.01 to 5.0 mole percent of at least one member selected from the group consisting of magnesium oxide (MgO), calcium oxide (CaO), barium oxide (B210) and strontium oxide (SrO), and 0.1 to 3.0 mole percent of manganese oxide (MnO).

According to the present invention, the n-value and stability with DC. load and surge power can be further improved and the C-value can be controlled when said additive consists essentially of 0.1 to 3.0 mole percent of bismuth oxide (B50 0.] to 3.0 mole percent of cobalt oxide (C0 0 0.01 to 5.0 mole percent of boron trioxide (B 03) and 0.01 to 5.0 mole percent of at least one member selected from the group consisting of magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO) and strontium oxide (SrO), 0.1 3.0 mole percent of manganese oxide (MnO) and one member selected from the group consisting of 0.05 to 3.0 mole percent of antimony oxide (Sb O and 0.] to 3.0 mole percent of titanium oxide (TiO The addition of antimony oxide increases the C-value of the resultant voltage-nonlinear resistor and the addtion of titanium oxide lowers the C-value of the resultant voltage nonlinear resistor.

According to the present invention, the stability with DC. load and the stability for surge pulses can be remarkably improved when said sintered body comprises, as a main constituent, zinc oxide (ZnO) and, as an additive, 0.1 to 3.0 mole percent of bismuth oxide (Blzog), 0.1 to 3.0 mole percent of cobalt oxide (C0 0 0.1 to 3.0 mole percent of manganese oxide (MnO), 0.01 to 5.0 mole percent of boron trioxide (B 0 0.] to 3.0 mole percent of titanium oxide (TiO- at least one member selected from the group consisting of 0.01 to 3.0 mole percent of chromium oxide (Cr O and 0.1 to 3.0 mole percent of nickel oxide (MO), and 0.0] to 5.0 mole percent of at least one member selected from the group consisting of magnesium oxide (MgO), calcium oxide (CaO), barium oxide (B210) and strontium oxide (SrO).

The sintered body I can be prepared by a per se well known ceramic technique. The starting materials in the compositions in the foregoing description are mixed in a wet mill so as to produce homogeneous mixtures. The mixtures are dried and pressed in a mold into desired shapes at a pressure from 50 Kg./cm to 500 Kg./cm

The pressed bodies are sintered in air at L000" to.

l,450C for l to 20 hours, and then furnace-cooled to room temperature (about C to about 30C). The mixtures can be preliminarily calcined at 700 to 1,000C and pulverized for easy fabrication in the subsequent pressing step. The mixture to be pressed can be admixed with a suitable binder such as water, polyvinyl alcohol, etc. It is advantageous that the sintered body he lapped at the opposite surfaces by abrasive powder such as silicon carbide in a particle size of 50a in mean diameter to H) p. in mean diameter. The sintered bodies are provided, at the opposite surfaces thereof with electrodes in any available and suitable method such as silver painting, vacuum evaporation or flame spraying of metal such as Al, Zn, Sn etc.

The voltage-nonlinear properties are not practically affected by the kind ofelectrodes used, but are affected by the thickness of the sintered bodies. Particularly, the C-value varies in proportion to the thickness of the sintered bodies, while the n-value is almost independent of the thickness. This surely means that the voltage nonlinear property is due to the bulk itself, but not to the electrodes.

Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent in order to connect the lead wires to the electrodes. Voltage-nonlinear resistors according to this invention have a high stability to temperature, for the DC. load test, which is carried out by applying a rating power of 1 watt at C ambient temperature for 500 hours, and for the surge test, which is carried out by applying surge wave form of 8 X 20p.sec, 500A/cm The n-value do not change remarkably after the. heating cycles, the load life test, humidity test, and surge test. Particularly, the C-value and n-value do not change so mucheven in a region of the current less than 0.lmA after DC. load life test. It is advantageous for achievement of a high stability with respect to humidity that the resultant voltage-nonlinear resistors be embedded in a humidity proof resin such as epoxy resin and phenol resin in a per se well known manner. Presently preferred illustrative embodiments of the invention are as follows.

EXAMPLE 1 Starting material composed of 97.0 mole percent of zinc oxide, l.0 mole percent of bismuth oxide, 10 mole percent of cobalt oxide, and 0.5 mole percent of boron trioxide and 0.5 mole percent of magnesium oxide is mixed in a wet mill for 24 hours. The mixture is dried and pressed in a mold into discsof 17.5mm in diameter and 7mm in thickness at a pressure of 250 Kg/cm".

The pressed bodies are sintered in air at the condition shown in Table l, and then furnace-cooled to room temperature. The sintered body is lapped at the opposite surfaces'thereof into the thickness shown in Table l by silicon carbide abrasive in particle size of 30 p. in mean diameter. The opposite surfaces of the sintered body are provided with a spray metallized film of aluminum in a per se well known technique.

The electric characteristics of resultant sintered body are shown in Table l, which shows that the C-value varies approximately in proportion to the thickness of the sintered body while the n-value is essentially independent of the thickness. It will be readily realized that the voltage-nonlinear property of the sintered body is attributed to the sintered body itself.

EXAMPLE 2 Zinc oxide and with additives listed in Table 2 are fabricated into voltage-nonlinear resistors by the same process as that of Example I. The thickness is 1.0 mm. The resulting electrical properties are shown in Table 2, in which the value of n are the n-values defined between 0.lmA and lmA. The test is carried out by appling D.C. load of l watt at 70C ambient temperature for 500 hours. It can be easily understood that the combined addition of bismuth oxide, cobalt oxide, boron EXAMPLE 3 Z n xisieaadadd l ves Iablc 3.3m fabricated into the voltage-nonlinear resistors by the same process as that of Example 2. The electrical properties of the resultant resistors are shown in Table 3. The change rates ofC and n values after D.C. load and impulse test are also shown in Table 3. The impulse test is carried out by applying two impulses of 8 X 20 usec, 500A, and D.C. load life test is carried out by the same method as that of Example 2. It will be readily realized that the further addition of manganese oxide results in the higher n-value and smaller change rates than those of Example 2.

EXAMPLE 4 Zinc oxide and additives of Table 4 are fabricated into the voltage-nonlinear resistors by the same process as Example 2. The electrical characteristics of resulting resistors are shown in Table 4. It will be easily understood that the further addition of one member selected from the group consisting of antimony oxide and titanium oxide results in the higher n-value and smaller change rates than those of Example 3. The change rates of C and n values after D.C. and impulse test carried out by the same method as that of Example 3 are also shown in Table 4.

EXAMPLE 5 Zinc oxide and additives of Table 5 are fabricated into the voltage-nonlinear resistors by the same process as Example 2. The electrical characteristics of resultant resistors are shown in Table 5. it will be easily understood that the further addition of nickel oxide and chromium oxide results in the higher n-value and smaller change rates than those of Example 4. The change rates of C and n values after D.C. test and im- Additives (mole pulse test carried out by the same method as those of Example 3 are also shown in Table 5.

EXAMPLE 6 The resistors of Example 2,3,4 and 5 are tested in accordance with a method widely used in the electronic component parts. The heating cycle test is carried out by repeating five times the cycle in which said resistors are kept at 85C ambient temperature for 30 minutes.

10 cooled rapidly to 20C and then kept at such temperature for 30 minutes. The humidity test is carried out at 40C and 95 percent relative humidity for 1,000 hrs. Table 6 shows the average change rates of C-value and n-value of resistors after heating cycle test and humidity test. It is easily understood that each sample has a small change rate.

Table 6 Heating Cycle Test Humidity Tcst (7r Sample No.

Example 2 4.9 6.5 5.2 68 Example 3 28 5.7 3.7 -5.4 Example 4 1.8 3.9 l .1 3.7 Example 5 1.0 2.5 0.8 2.2

Table l 30 Thickness c Sintcring (mm) (at lmA) n Condition initial (5) 680 24 1200C 5 Hours 2 275 24 do. 1 135 23 do. 0.5 68 23 do.

initial (5) 600 24 1350C. 1 Hour 2 240 24 do. l 121) 23 do. 0.5 23 do.

initial (5) 850 23 1000C. 20 Hours 40 2 330 23 do. 1 170 23 do. 0.5 23 do.

Table 2 Electrical Properties Change Rate after of Resultant Resistor Test c c Bi,0 C0 0 B 0 MgO at l mA n at 0.01 n

0.01 3.0 0.5 0.5 57 20 4.3 4.3 0.1 3.0 0.5 0.5 79 20 2.6 2.1 1.0 3.0 0.5 0.5 133 23 2.4 2.3 3.0 3.0 0.5. 0.5 141 22 2.5 2.4 10.0 3.0 0.5 0.5 149 22 4.9 4.4 3.0 0.01 0.5 0.5 69 18 4.8 4.5 3.0 0.1 0.5 0.5 20 2.4 2.2 3.0 1.0 0.5 0.5 129 22 2.1 2.3 3.0 3.0 0.5 0.5 137 20 2.7 2.6 3.0 10.0 0.5 0.5 158 19 4.9 5.0 3.0 3.0 0.01 0.5 117 25 2.9 2.5 3.0 3.0 0.1 6.5 125 23 2.7 2.4 3.0 3.0 1.0 0.5 123 26 2.3 2.4 3.0 3.0 5.0 0.5 142 27 --2.6 2.5 3.0 3.0 0.5 0.01 137 25 2.8 2.9 3.0 3.0 0.5 0.1 126 23 2.7 2 2 3.0 3.0 0.5 1.0 122 23 2.6 2.4 3.0 3.0 0.5 5.0 106 25 2.7 2.4 Bi,0 C0 0 8 0 C210 Table 2 Continued Change Rate after est atlmA Bi O, C0,O

5053 6 9 0 .s .95 .s .1 3A .5 .5 .3 .5 .5

50J0osssjnwfijjjjjjssfinlonn000%555555555501005555 l l 1 ll llll 0! (I.(O SOOOO OOOOO0000000 53333 0O0O0OO0OO0( l a l .nonnnnooonfioonnnnl000000000000000000100000000000 l 1 I DDODODDDAWDJDDDDDDDDDHDDDDDDQDJDDDDDDDDDD000000 Table 3 Electrical Properties of Change Rate after Change Rate after Resultant Resistor DC Life Test Impulse Test Additives (mole c at 0.005mA 8,0; MnO Further c at a given additives current of l mA omfiwomomomomomomom Table 3 Continued Additives (moleflu) Electrical Properties of Change Rate ifter Chan e Rate after Resultant 'Reliltor DC Life Ten 1mpu1ee Tes1(%) cop, 8,0, M110 F1 |rt11er c at a given addltwel current of 1 mA at 0.005mA 1 00 9342 2 225 22 917.3 2 .971 2 7343460 00 908125.300 321357.129 mu n .7 lmnumnmmn mnnmmnmnu w ww 444444444 444444444 444444444 444444444 4444 m m 6H c r Ca C 7 3 .1 .2 .39 2 .01 0 3 .1 .2 .3 2 .81 2 8 .13 5 8 20 4 09 6 8 666666666 66 66666 6%6777717 7777. 7 6 66 66 65 098099099988 9898un A. 1, H 1 u D #0 .28 .98 .3 01 0 .929 2 .12201 12 .01 03 9 .0 5 5 .3 .5 .15A 42 244 0 .3 na 2 2 2 221 1 2 2 222222212 ZZZZZLZZZ 1222 2 1 222222222 222222222 Ma A f ai m R 2 C 60 M 0 D 0. 861883058 .A .3 .09 .12 6 .58 .88 0 2 .8 .0 .00 00298977 87066654 111111 11 11 111111 111 1 11 1 1 1 1 l 1 1 11 11111 11 .MZZZLLLLLMLLZLHLLL 2 m n 8m 0 z l mm 0232 6034 4 2 9020249 3 9 2 2 923 2022222 "HUB 555555555 555555 5 455555455 45555 455 335311 3 S 794 82505 7046835 5 mmfle 223223233 233223233 HORR f 1A P m C 1 062 767-89 97 267 98 850055077 860 56087 20076Uufi 9 558568780 458568780 458568780 458568780 2 2 512 1 1 1 1 4 0 am 111 111 111 111 I11 wmwawnumu wmmwwnumu w mm 000000000 000000000 000000000 000000000 000000000 A 11 1 11 11 1 11 T m 000 1333 0001 333 000 333 000 1 333 000 1 5. F A 000000000 000 ggggggggg wwwwwwwwm wwwwwwwww mmmmmmmmm mmmmmm M MMMMMMMM CCCCCCCCC BBBBBBBBB SSSSSSSSS MMMMMMMMM 1.11.. 1.1 1. 2 000000000 000000000 0 )1 11 v1 00 555 000 555 T 111111111 0 l 3 O wwwwwwwmw 000000000 5 0 5 5 5 5 5 5 5 5 5 5 5 5 BBBBBBBB SSSSSSSSS m S 0 0 .00000 00000 .005 00000000 000000000 2 H. m 0 3013013 0 30 30 3 0 30130 3 0 30 30 3 m s 1 .01 501 5 1 .01 .01 5 w w 1 003003003 003003003 m M 111555000 111555000 111555000 111555000 111555000 M 000000333 000000333 000000333 000000333 00000 3 A 2 111000000 111000000 0 000111555 000111555 R 11100000 11100 .00 1 .1 0.0 .0 11 1 .0 .00 11100 .000" 000 555 000 333 000 333 000 333 000 3 3 111555000 111555000 000000333 000000333 0 11155500 1 .155 5 .0 1 .1 .5 .00 .11 5 .5 .00 1 .1555000 C 0000 0333 000000333 000000333 000000333 00000 1 2 M a mmmmmmmmmm mmm uuu m 111555000 111555000 111555000 111555000 111555000 ((1 B 000000333 000000333 000000333 000000333 0000003 1.

TABLE -Continued Additives (mole percent) Ac at 71 0.001 ma.

Further B120: C0203 MnO T102 B203 N ClzOa additives Atlma.

100899870 ZZZLLLLLZ 000LLL003 00000000 8838 388 CCCCCCCC M50m50 422210652 xmidfxdfa a xmd 211000081 ZZZLZZLLQ 0 0 5 zmammmmam 00 0LLL003 111000000 0 0 0 LLL3 3 3 099887655 LL LOLLLL 000008088 ZZLLLLLLL ush 40585 11311323 0 00LLL330 111000000 0 0 0 1LL03J3 0.5 3.0 CBO 0.5 SrO What we claim is: 1. A voltage centof boron trioxide (B 03) and 0.01 to 5.0 mole perli resistor f th b lk t 55 cent of at least one member selected from the group prising a sintered body consisting essentially of, as a consisting of magnesium oxide (MgO). calcium oxide (CaO), barium oxide (BaO) and. strontium oxide (SrO).

major part, zinc oxide (ZnO) and-. as an additive, 0.01 to 10.0 mole percent of bismuth oxide (Bi O 0.01 to 3. Voltage-nonlinear resistor according to claim 2, 0 wherein said sintered body further includes, as an addi- 10.() mole percent of cobalt oxide (C0 0 0.01 to 5.0 mole percent of boron trioxide (B 0 and 0.01 to 5.0 mole percent of at least one member selected from the group consisting of magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO) and strontium oxide (SrO) and electrodes in contact with said body.

tive, 0.1 to 3.0 mole percent of manganese oxide (MnO).

4. Voltage-nonlinear resistor according to claim 3,

wherein said sinteredbody further includes, as an addi- 2. Voltage-nonlinear resistor according to claim 1, live one member Selected from the group consisting of wherein said additive consists essentially of 0.1 to 3.0 l0 mole P 0f ly Oxide zon) mole percent of bismuth oxid (M 0 0,] t 3 l and 0.1 to 3.0.mole percent of titanium oxide (TiO percent of cobalt oxide (C0 0 0.01 to 5.0 mole per- 5. Voltage-nonlinear resistor according to claim 3,

(Cr O and 0.] to 3.0 mole percent of nickel oxide (NiO); 

1. A VOLTAGE-NONLINEAR RESISTOR OF THE BULK TYPE COMPRISING A SINTERED BODY CONSISTING ESSENTIALLY OF, AS A MAJOR PART, ZINC OXIDE (ZNO) AND, AS AN ADDITIVE, 0.01 TO 10.0 MOLE PERCENT OF BISMUTH OXIDE (BI2O3), 0.01 TO 10.0 MOLE PERCENT OF COBALT OXIDE (CO203), 0.01 TO 5.0 MOLE PERCENT OF BORON TRIOXIDE (B2O3) AND 0.01 TO 5.0 MOLE PERCENT OF AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM OXIDE (MGO), CALCIUM OXIDE (CAO), BARIUM OXIDE (BAO) AND STRONTRIUM OXIDE (SRO) AND ELECTRODES IN CONTACT WITH SAID BODY.
 2. Voltage-nonlinear resistor according to claim 1, wherein said additive consists essentially of 0.1 to 3.0 mole percent of bismuth oxide (Bi2O3), 0.1 to 3.0 mole percent of cobalt oxide (Co2O3), 0.01 to 5.0 mole percent of boron trioxide (B2O3) and 0.01 to 5.0 mole percent of at least one member selected from the group consisting of magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO) and strontium oxide (SrO).
 3. Voltage-nonlinear resistor according to claim 2, wherein said sintered body further includes, as an additive, 0.1 to 3.0 mole percent of manganese oxide (MnO).
 4. Voltage-nonlinear resistor according to claim 3, Wherein said sintered body further includes, as an additive, one member selected from the group consisting of 0.05 to 3.0 mole percent of antimony oxide (Sb2O3) and 0.1 to 3.0 mole percent of titanium oxide (TiO2).
 5. Voltage-nonlinear resistor according to claim 3, wherein said sintered body further includes, as an additive, 0.1 to 3.0 mole percent of titanium oxide (TiO2) and at least one member selected from the group consisting of 0.01 to 3.0 mole percent of chromium oxide (Cr2O3) and 0.1 to 3.0 mole percent of nickel oxide (NiO). 